Hartsfield-Jackson Atlanta International Airport wrested the top honor away from O'Hare in 2005 and had retained it since, according to the official flight count by the Federal Aviation Administration.

While 2014 traffic at O'Hare was down by 0.2 percent, it lost less traffic than the Atlanta airport, where traffic declined by 4.7 percent, according to new FAA data. O'Hare had 881,933 arrivals and departures; Atlanta had 868,359.

But the Atlanta airport is likely to retain the title of busiest airport for passenger traffic.

City officials have said international passenger volume have helped O'Hare totals. In the last 18 months, O'Hare and Midway airports added six new international airlines and dozens of destinations, according to the city.

O'Hare regains bragging rights to the title it had mostly owned since the dawn of the Jet Age, when it surpassed the number of flights at Midway Airport, which had been the leader.

O'Hare might be the busiest airport, but during some months it's also regularly among the worst for flights departing on time, according to data from the U.S. Department of Transportation.